Researchers explore one-directional sound waves

In a recent study, Yale researchers developed a new technology that transmits sound in one direction.

Researchers in the Yale physics department were able to restrict audio waves to one direction by controlling how they interfere with each other. The study, published in Nature on April 3, could lead to changes in the way digital information is processed and the way quantum data is stored. The findings also demonstrate how easily one-way sound transmission can be achieved using preexisting technologies such as oscillators with parametric coupling.

“There’s a very familiar class of devices that is just ready to do this,” said Jack Harris, Yale professor of physics and lead author of the study. “What we did was to find this thing that’s already in the basic tool kit of every engineer and show that if you rearrange it in a slightly different way, bang, you get one-way flow of sound waves.”

The study stemmed from an earlier collaboration between Harris’s team and Aashish Clerk, a theoretical physicist at the University of Chicago. Their previous study prompted them to brainstorm general ideas about how to achieve unidirectional sound flow.

After developing a theory, Harris and Clerk realized they could produce this type of sound wave using a laser system housed in the basement of Yale’s Sloane Physics Laboratory. In the new study, the researchers trapped light from the laser between two mirrors and placed a vibrating membrane that produced sound waves between the mirrors in the light’s pathway. By controlling the phase of the laser, they were able to modify the interference of the sound waves, or the way the waves combine to cancel or strengthen each other, explained Haitan Xu, a Yale postdoctoral associate and co-author of the study.

The scientists next applied this technology to transfer heat — which can be produced by sound vibrations. By directing sound waves, the researchers were able to make heat flow from one object to another regardless of their relative temperatures. While Harris said that this study focuses on the research technique behind producing unidirectional sound, the group is also interested in the practical applications of the findings. For example, the results might change the way information is transmitted in telecommunication devices, Harris said.

“In your cell phone, there are probably a couple dozen places where the signal gets converted from electricity to sound waves,” he added. “That happens despite the fact that the engineers lack a way of making sure that sound is only going in one direction through your cell phone chip. They just have to live with the fact that sound waves are bouncing back and forth and coming the wrong way through the device, leading to all kinds of noise and interference and unwanted effects.”

Clerk said that the researchers are interested in extending the experiment to the quantum level. If the temperature of the membrane were lowered enough, classical mechanics would no longer be able to describe its motion and the sound waves it produces. The one-way flow of the waves, however, should still work at this level.

“There’s a huge amount of interest in creating new technology that uses the weird aspects of quantum mechanics,” he said. “It could be useful for creating the funny kind of quantum states you need for quantum communications or quantum computing. It could even be a better way to read out quantum information.”

Grants from the Air Force, the Navy and the Simons Foundation funded the study.